In the prior art, the elastic means described above take the form of a series of Belleville washers arranged coaxially around the tool-holding collet, which are in contact with a special part of the collet, on which they exert a predetermined pressure due to the abovementioned precompression, keeping it closed during machining. If an axial force is applied to the tool-holding collet, via a rod passing through the interior of the spindle cavity containing the Belleville washers, the latter are further compressed to bring about this opening action of the collet.
However, the system described above has certain troublesome drawbacks: firstly, at the extremely high rotational speeds of the spindle that are currently achievable (over 100,000 rpm), the said Belleville washers are drawn round by the rotation, creating effects of unbalance which cause vibrations harmful to machining accuracy. Also, to accommodate Belleville washers with specifications suitable for the job, a large-diameter cavity must be formed in the spindle, which means that the spindle itself must have a large diameter and consequently a large mass.
Moreover, with the system currently in use, it is necessary, as already discussed, to operate from the outside with a rod which must move cyclically into contact with the end of the tool-holding collet and exert upon it a force (of the order of some 2,000 N) which, over time, owing in part to the situation of point loading that occurs, can deform the tip and/or alignment of the rod, causing problems of accuracy and service life.
In order to solve all the problems listed briefly above, the inventor of the present innovation has devised a spindle of the type defined above, in which the said elastic means are not Belleville washers but instead a sleeve of elastic material (rubber or another elastomer) which also, under conditions of precompression, keeps the tool-holding collet in the closed position as described. This sleeve is fitted onto the free end of an extension of the said tool-holding collet, which ends near the corresponding end of the spindle, and can be reached without having to insert, as discussed, a long rod down into the body of the spindle.
In order for the radial expansion of the said sleeve not to create braking forces on the tool-holding collet, there is installed between the collet and the sleeve a shaped steel or epoxy resin collar which, having a low coefficient of friction, does not significantly oppose the axial movement of the tool-holding collet.
For similar reasons there is interposed, between the body of the spindle and the tool-holding collet, in that part of its extension which is not surrounded by the abovementioned sleeve, an epoxy resin bush, which can be formed inside the spindle itself during the assembly operation and has a low coefficient of friction and permits movements in the axial direction of the tool-holding collet with virtually zero play.